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Date November 2021 Marks available 6 Reference code 21N.1.AHL.TZ0.9
Level Additional Higher Level Paper Paper 1 (without calculator) Time zone Time zone 0
Command term Find Question number 9 Adapted from N/A

Question

Consider the expression 11+ax-1-x where a, a0.

The binomial expansion of this expression, in ascending powers of x, as far as the term in x2 is 4bx+bx2, where b.

Find the value of a and the value of b.

[6]
a.

State the restriction which must be placed on x for this expansion to be valid.

[1]
b.

Markscheme

attempt to expand binomial with negative fractional power                 (M1)

11+ax=1+ax-12=1-ax2+3a2x28+                A1

1-x=1-x12=1-x2-x28+                A1

11+ax-1-x=1-a2x+3a2+18x2+

attempt to equate coefficients of x or x2                 (M1)

x :  1-a2=4b;  x2 : 3a2+18=b

attempt to solve simultaneously                 (M1)

a=-13, b=16                A1

 

[6 marks]

a.

x<1              A1

 

[1 mark]

b.

Examiners report

[N/A]
a.
[N/A]
b.

Syllabus sections

Topic 1—Number and algebra » AHL 1.10—Perms and combs, binomial with negative and fractional indices
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Topic 5 —Calculus » AHL 5.19—Maclaurin series
Topic 1—Number and algebra
Topic 5 —Calculus

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